In a virtual memory technique, when cache miss occurs, data is transferred from a secondary memory to a main memory. However, transfer processing takes time, and lowers the performance of the overall system.
In the virtual memory technique, “write back” is used to keep coherency between the main memory and the secondary memory. In other word, the write back is used to reduce dirty entries in the main memory. In a general operating system (OS) has a function of periodically check the dirty entries in the main memory and of writing back the data from the main memory to the secondary memory to keep coherency.
However, with conventional write back technique, the write back is performed with predetermined fixed time interval, for example, 30 seconds after the last (latest) write access.